On flat ground the normal force equals the weight, and friction is a fraction of that normal force resisting any slide.
Example
On flat ground the normal force equals the weight, and friction is a fraction of that normal force resisting any slide.
highlighted = computed this step
On flat ground the normal force equals the weight
A block of mass 2 kilograms rests on flat ground. Its weight is 2 times 10, or 20 newtons down, and the ground pushes back with an equal normal force of 20 newtons up.
N=mg=2kg⋅10m/s2=20N
Friction is proportional to the normal force
Friction acts sideways, resisting sliding, and it is proportional to how hard the surfaces are pressed together, which is the normal force. The friction is some fraction mu of the normal force. That fraction is a property of the two surfaces in contact, found by measurement, not a universal constant like gravity: rougher pairs have a bigger one. We use clean values here. The rule sets how big the friction can get; we split the still case from the sliding case next.
f=μN
mechanicsA 2 kg block on flat ground gives a clean 20 N normal force, and friction is just a fraction of it.